The present invention relates to a process for the preparation and/or working-up of polymerizable compounds and an apparatus which is used in this process. In the preparation of polymerizable compounds, e.g. (meth)acrylates, styrene, vinyl acetate, etc., in general at least one process step is carried out under reduced pressure, for example the working-up by distillation or the preparation itself.
Thus, Ullmann""s Encyclopedia of Industrial Chemistry, Sixth Edition, 1999, Electronic Release, Section Acrylic Acid and Derivativesxe2x80x94Esterification describes processes for the preparation of higher and lower alkyl acrylates by acid-catalyzed esterification of acrylic acid with the corresponding alcohol in the presence of an acidic catalyst, in which distillation columns in the working-up of the reaction mixture are operated under reduced pressure in order to be able to reduce the distillation temperatures. Such a reduction in the temperature to which polymerizable compounds are exposed is desirable since the danger of polymerization generally increases with increasing temperatures.
In chemical process engineering, liquid ring pumps, in particular water ring pumps, as described, for example, in the above-mentioned Ullmann""s Encyclopedia, in the section Vacuum Technologyxe2x80x94Liquid and Gas Ring Vacuum Pumps, are widely used for generating a vacuum.
Such pumps have an eccentrically mounted rotor with blades and a working liquid which is present in the casing and forms an annular liquid layer during rotation of the impeller on the inside of the casing.
Because of the blades, this liquid ring forms separate cells which, owing to the eccentricity of the rotor with respect to the axis about which the liquid ring is arranged, have different volumes and thus aspirate a gas volume during rotation of the rotor on the suction side of the pump and convey the gas volume to the outlet side.
Since some working liquid is also discharged on the outlet side, it is generally necessary to compensate the loss by feeding in fresh working liquid.
With the usual use of water as a working liquid, it is possible to generate a vacuum of less than 30 mbar (depending on the water temperature). Since liquid ring pumps can process up to about 5% of their working volume of impurities or liquid drops, they can also be used as condensers.
The disadvantage of pumps of this type is that polymerizable material sucked into the pump can polymerize there and leave behind deposits and is in any case lost to the preparation process since working-up of the discharge is uneconomical owing to the large amounts of water.
EP-A 1 057 804describes a process for inhibiting polymerization in the vacuum part in the working-up of a readily polymerizable compound by feeding a polymerization inhibitor into the vacuum part.
The disadvantage of the process described there is that no working-up of the polymerizable compound present in the vacuum part is intended and hence the polymerization inhibitor fed in is lost.
It is an object of the present invention to provide a process which reduces the polymerization of polymerizable compounds in liquid ring pumps and reduces the losses of desired product via the liquid ring pump.
We have found that this object is achieved by a process for the preparation and/or working-up of polymerizable compounds using at least one liquid ring pump, in which a working liquid which contains a material stream from the preparation and/or working-up of the polymerizable compound is used in a liquid ring pump.
The novel process can be used in the preparation and/or working-up of polymerizable compounds, for example styrene, vinyl acetate, vinyl propionate, allyl acetic acid, vinylacetic acid, N-vinylformamide, acrylic acid or methacrylic acid (referred to in this document as (meth)acrylic acid) and preferably (meth)acrylates, such as methyl, ethyl, n-butyl and particularly preferably 2-ethylhexyl (meth)acrylate.
Liquid ring pumps which may be used are commercial liquid ring pumps which are known per se to a person skilled in the art and whose working liquid contains a material stream from the preparation and/or working-up of the polymerizable compound.
The working liquid preferably comprises at least 50, particularly preferably at least 75, very particularly preferably 100, % by weight of one or more of said material streams and can, if required, be supplemented by another compound suitable as working liquid, e.g. water.
For example, the pure products, i.e. the polymerizable compounds, in a purity of, as a rule, 95% or more, preferably 98% or more, particularly preferably 99% or more, can be used as the material stream from the preparation and/or working-up, but the starting materials, in a purity of 95% or more, preferably 98% or more, particularly preferably 99% or more, which are used for the preparation of the polymerizable compound, or those material streams which contain the starting materials and/or products and/or intermediates and/or byproducts, may also be used.
This may be, for example, in the case of
Styrene: Benzene or styrene in the above purities or styrene-containing distillation bottom products,
Vinyl acetate: Acetic acid or vinyl acetate in the above purities,
Vinyl propionate: Propionic acid or vinyl propionate,
(Meth)acrylic acid: (Meth)acrylic acid in the above purity, distillation bottom products containing it, streams containing Michael adducts of (meth)acrylic acid, e.g. diacrylic acid, triacrylic acid, etc., high-boiling absorbents, e.g. diphenyl ether, biphenyl, dimethyl, diethyl, di-n-butyl, diisobutyl or di-2-ethylhexyl orthophthalate or mixtures thereof,
(Meth)acrylates: (Meth)acrylic acid, starting alcohol, (meth)acrylates in the above purity, distillation bottom products containing them, streams containing Michael adducts of (meth)acrylic acid or its esters, e.g. alkyl alkoxypropionate, alkyl 3-(3xe2x80x2-alkoxypropionyloxy)propionate, etc., solvents which form an azeotropic mixture with water, the (meth)acrylate and/or the alcohol, e.g. cyclohexane, methyl cyclohexane, benzene, toluene, o-, m- or p-xylene or mixtures thereof.
In a preferred embodiment, one or more polymerization inhibitors are mixed with the working liquid.
Examples are alkylphenols, for example o-, m- or p-cresol (methylphenol), 2-tert-butyl-4-methylphenol, 6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol or 2,2xe2x80x2-methylenebis(6-tert-butyl-4-methylphenol), 4,4xe2x80x2-oxydiphenyl, 3,4-methylenedioxydiphenol (sesamol), 3,4-dimethylphenol, hydroquinone, pyrocatechol (1,2-dihydroxybenzene), 2-(1xe2x80x2-methylcyclohex-1xe2x80x2-yl)-4,6-dimethylphenol, 2- or 4-(1xe2x80x2-phenyleth-1xe2x80x2-yl)phenol, 2-tert-butyl-6-methylphenol, 2,4,6-tris-tert-butylphenol, 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 4-tert-butylphenol, nonylphenol [11066-49-2], octylphenol [140-66-9], 2,6-dimethylphenol, bisphenol A, bisphenol F, bisphenol B, bisphenol C, bisphenol S, 3,3,5,5xe2x80x2-tetrabromobisphenol A, 2,6-di-tert-butyl-p-cresol, Koresin(copyright) from BASF AG, methyl 3,5-di-tert-butyl-4-hydroxybenzoate, 4-tert-butylpyrocatechol, 2-hydroxybenzyl alcohol, 2-methoxy-4-methylphenol, 2,3,6-trimethylphenol, 2,4,5-trimethylphenol, 2,4,6-trimethylphenol, 2-isopropylphenol, 4-isopropylphenol, 6-isopropyl-m-cresol, n-octadecyt-p-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,1,3-tris(2-methyl-4-hydroxyl-5-tert-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tis(3,5-di-tert-butyl-4hydroxybenzyl)benzene, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethyl isocyanurate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate or pentaerythrityl tetrakis[xcex2-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,6-di-tert-butyl-4-dimethylaminomethylphenol, 6-sec-butyl-2,4-dinitrophenol, Irganox(copyright) 565, 1141, 1192, 1222and 1425 from Ciba Spezialitatenchemie, octadecyl 3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate, hexadecyl 3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate, octyl 3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate, 3-thia-1,5-pentanediol bis[(3xe2x80x2,5xe2x80x2-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate], 4,8-dioxa-1,11-undecanediol bis[(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate], 4,8-dioxa-1,11-undecanediol bis[(3xe2x80x2-tert-butyl-4xe2x80x2-hydroxy-5xe2x80x2-methylphenyl)propionate], 1,9-nonanediol bis[(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionate], 1,7-heptanediaminebis[3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionamide], 1,1-methanediaminebis[3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionamide], 3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl)propionic acid hydrazide, 3-(3xe2x80x2,5xe2x80x2-dimethyl-4xe2x80x2-hydroxyphenyl)propionic acid hydrazide, bis(3-tert-butyl-5-ethyl-2-hydroxyphen-1-yl)methane, bis(3,5-di-tert-butyl-4-hydroxyphen-1-yl)methane, bis[3-(1xe2x80x2-methylcyctohex-1xe2x80x2-yl)-5-methyl-2-hydroxyphen-1-yl]-methane, bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)methane, 1,1-bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)ethane, bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)sulfide, bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)sulfide, 1,1-bis(3,4-dimethyl-2-hydroxyphen-1-yl)-2-methylpropane, 1,1-bis(5-tert-butyl-3-methyl-2-hydroxyphen-1-yl)butane, 1,3,5-tris[1xe2x80x2-(3xe2x80x3,5xe2x80x3-di-tert-butyl-4xe2x80x3-hydroxyphen-1xe2x80x3-yl)meth-1xe2x80x2-yl]-2,4,6-trimethylbenzene, 1,1,4-tris(5xe2x80x2-tert-butyl-4xe2x80x2-hydroxy-2xe2x80x2-methylphen-1xe2x80x2-yl)butane, aminophenols, e.g. para-aminophenol, nitrosophenols, e.g. para-nitrosophenol or p-nitroso-o-cresol, alkbxyphenols, for example 2-methoxyphenol (guajacol, pyrocatechol monomethyl ether), 2-ethoxyphenol, 2-isopropoxyphenol, 4-methoxyphenol (hydroquinone monomethyl ether), mono- or di-tert-butyl-4-methoxyphenol, 3,5-di-tert-butyl-4-hydroxyanisole, 3-hydroxy-4-methoxybenzyl alcohol, 2,5-dimethoxy-4-hydroxybenzyl alcohol (syringa alcohol), 4-hydroxy-3-methoxybenzaldehyde (vanillin), 4-hydroxy-3-ethoxybenzaldehyde (ethylvanillin), 3-hydroxy-4-methoxybenzaldehyde (isovanillin), 1-(4-hydroxy-3-methoxyphenyl)ethanone (acetovanillone), eugenol, dihydroeugenol or isoeugenol, tocopherols, e.g. xcex1-, xcex2-, xcex3-, xcex4- and xcex5-tocopherol, tocol, xcex1-tocopherolhydroquinone, and 2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran (2,2-dimethyl-7-hydroxycoumarane), quinones and hydroquinones, such as hydroquinone or hydroquinone monomethyl ether, 2,5-di-tert-butyl hydroquinone, 2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone, trimethylhydroquinonc, 4-methylpyrocatechol, tert-butylhydroquinone, 3-methylpyrocatechol, benzoquinone, 2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone, trimethylhydroquinonc, 3-methylpyrocatechol, 4-methylpyrocatechol, tert-butylhydroquinone, 4-ethoxyphenol, 4-butoxyphenol, hydroquinone monobenzyl ether, p-phenoxyphenol, 2-methylhydroquinone, 2,5-di-tert-butylhydroquinone, tetranethyl-p-benzoquinone, diethyl-1,4-cyclohexanedione-2,5-dicarboxylate, phenyl-p-benzoquinone, 2,5-dimethyl-3-benzyl-p-benzoquinone, 2-isopropyl-5-methyl-p-benzoquinone (thymoquinone), 2,6-diisopropyl-p-benzoquinone, 2,5-dimethyl-3-hydroxy-p-benzoquinone, 2,5-dihydroxy-p-benzoquinone, embelin, tetrahydroxy-p-benzoquinone, 2,5-dimethoxy-1,4-benzoquinone, 2-amino-5-methyl-p-benzoquinone, 2,5-bisphenylamino-1,4-benzoquinone, 5,8-dihydroxy-1,4-naphthoquinone, 2-anilino-1,4-naphthoquinone, anthraquinone, N,N-dimethylindoaniline, N,N-diphenyl-p-benzoquinonediimine, 1,4-benzoquinonedioxime, coerulignone, 3,3xe2x80x2-di-tert-butyl-5,5xe2x80x2-dimethyldiphenoquinone, p-rosolic acid (aurin), 2,6-di-tert-butyl-4-benzylidenebenzoquinone, 2,5-di-tert-amylhydroquinone, N-oxyls, such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl, 4-oxo-2,2,6,6-tetramethylpiperidin-N-oxyl, 4-acetoxy-2,2,6,6-tetramethylpiperidin-N-oxyl, 2,2,6,6-tetramethylpiperidin-N-oxyl, 4,4xe2x80x2,4xe2x80x3-tris(2,2,6,6-tetramethylpiperidin-N-oxyl)phosphite, 3-oxo-2,2,5,5-tetramethylpyrrolidin-N-oxyl, 1-oxyl-2,2,6,6-tetramethyl-4-methoxypiperidine, 1-oxyl-2,2,6,6-tetramethyl-4-trimethylsilyloxypiperidine, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-2-ethylhexanoate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-stearate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-benzoate, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-(4-tert-butyl)benzoate, bis(1-oxyl-2,2,6,6-tctramethylpiperidin-4-yl)succinate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl) 1,10-decanedioate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)n-butylmalonate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)phthalate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)isophthalate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)terephthalate, bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)hexahydroterephthalate, N,Nxe2x80x2-bis(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)adipamide, N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)caprolactam, N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)dodecylsuccinimide, 2,4,6-tris[N-butyl-N-(1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl]-triazine, N,Nxe2x80x2-bis( 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-N,Nxe2x80x2-bisformyl-1,6-diaminohexane, 4,4xe2x80x2-ethylenebis(1-oxyl-2,2,6,6-tetramethylpiperazin-3-one), aromatic amines or phenylenediamines, such as N,N-diphenylamine, N-nitrosodiphenylamiine, nitrosodiethylaniline, N,Nxe2x80x2-dialkyl-para-phenylendiamine, where the alkyl radicals may be identical or different and in each case, independently of one another, are of 1 to 4 carbon atoms and may be straight-chain or branched, for example N,Nxe2x80x2-diisobutyl-p-phenylendiamine, N,Nxe2x80x2-diisopropyl-p-phenylendiamine, frganox 5057 from Ciba Spezialitaitenchemie, p-phenylenediaminc, N-phenyl-p-phenylenediaamine, N,Nxe2x80x2-diphenyl-p-phenylenediamine, N-isopropyl-N-phenyl-p-phenylenediamine, N,Nxe2x80x2-di-sec-butyl-p-phenylenediamine (Kerobit(copyright) BPD from BASF AG), N-phenyl-Nxe2x80x2-isopropyl-p-phenylenediamine (Vulkanox(copyright) 4010 from Bayer AG), N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine, N-phenyl-2-naphthylamine, iminodibenzyl, N,Nxe2x80x2-diphenylbenzidine, N-phenyltetraaniline, acridone, 3-bydroxydiphenylamine, 4-hydroxydiphenylamine, hydroxylamines, such as N,N-diethylhydroxylamine, urea derivatives, such as urea or thiourea, phosphorus-containing compounds, such as triphenylphosphine, triphenyl phosphite, hypophosphorous acid or triethyl phosphite, or sulfur-containing compounds, such as diphenyl sulfide or phenothiazine.
They may be used, if required, as a mixture with metal salts, for example copper, manganese, cerium, nickel or chromium salts as chloride, dithiocarbamate, sulfate, salicylate or acetate.
4-Hydroxy-2,2,6,6-tetramethylpiperidin-N-oxyl, hydroquinone monomethyl ether or phenothiazine is preferably used.
The polymerization inhibitors are each usually used in amounts of from 10-2000 ppm, preferably 20-1000 ppm, particularly preferably 50-500 ppm.
It is of course also possible to use larger amounts of polymerization inhibitors, which are limited only by the solubility of the inhibitor in the solvent, for example 0.2-5, preferably 0.3-3, particularly preferably 0.5-2, % by weight.
In a particularly preferred embodiment, the stabilizer batches are used as working liquid, i.e. a solution of at least one polymerization inhibitor in one of said material streams, preferably in a starting material or product, which are prepared for metering the polymerization inhibitor into the preparation or working-up process.
In general, those material streams which have a flashpoint of 55xc2x0 C. or higher, particularly preferably 65xc2x0 C. or higher, are preferably used as working liquids. This is advisable for safety reasons, if leaks should occur in a liquid ring pump, in order, for example, to avoid fires.